This application proposes to enhance the development of a unique fish model that allows study of both environmental and genetic determinants of carcinogenesis, namely Xiphophorus genetic hybrids. A variety of spontaneous and induced tumors are available in hybrids produced by matings between different Xiphophorus (Teleostei: Poeciliidae) species. Although a few of these hybrids have been studied for many years, and have provided a useful nonmammalian animal model for tumorigenesis (primarily melanoma), most have only been preliminarily characterized. Therefore, this application has two central goals: the experimental development of chemical (N-methyl-N-nitrosourea, MNU)-induced tumor models to provide comparison between MNU-induced tumorigenesis and spontaneous and ultraviolet light (UV)-induced tumorigenesis (Specific Aims 1 and 2); and the development of molecular biological tools and reagents necessary to enhance the utility and relevance of Xiphophorus hybrids as nonmammalian disease models (Specific Aim 3). In the first two Specific Aims, the applicants will use both classical and molecular genetic approaches to analyze MNU-induced tumor susceptibility in backcross hybrid fish produced by two interspecific matings: X. couchianus x (X. maculatus Jp 163A x X. couchianus), and X. couchianus x (X. maculatus Jp 163B x X. couchianus). Backcross hybrids produced from these crosses are predisposed to MNU-induced melanomas and retinoblastomas. The parental stocks utilized in these crosses carry over 40 fixed polymorphic genetic markers, allowing the applicants to perform joint segregation analyses of chromosomal inheritance with induced tumor susceptibility. These cosegregation analyses will enable assignment of map locations to loci associated with, and/or directly involved in, tumor susceptibility. The research proposed for Specific Aim 3 will enhance the experimental utility and relevance of Xiphophorus genetic hybrids as disease models by providing extensive gene map saturation and establishing characterized molecular libraries. Map saturation, coupled with the molecular tools to be developed and characterized in this project [Xiphophorus yeast artificial chromosome (YAC) and cosmid DNA libraries], will provide investigators with an experimental system enabling the isolation of genes showing genetic association with any phenotype of interest.